The present invention relates generally to form keys and a method for using form keys for thermoforming undercuts in matched molded foam parts. More particularly, the invention relates to form keys for forming a reverse draft in thermoformed parts, and to a method for thermoforming undercuts in parts using matched metal molds and for stripping the part from the molds while maintaining the undercuts.
Foam containers for restaurants, schools, hospitals and the like are typically formed by placing a foam sheet between a pair of molds and closing the molds to thermoform a part. If the part requires, for example, a lock for a hinged lid container, a reverse draft or undercut can be formed, but forming a reverse draft or undercut for a lock in the part, particularly foam parts, has been a problem. A reverse draft has been difficult to form in foam containers such as polystyrene foam containers due to an inability to strip the formed part from the tooling while maintaining the desired shape of the reverse draft. For this reason, a continuous thermoforming process for forming parts, with a reverse draft has not been possible. If this problem could be overcome, this would allow for different closure designs in parts, and the development of continuous thermoforming processes for forming foam parts.
The present invention is directed to form keys for forming a reverse draft in a thermoformed part, for example, a polystyrene foam part such as a hinged lid container, and to a method for thermoforming a reverse draft or undercut in the part and stripping the part from a mold while maintaining the shape of a reverse draft or undercut. The form key is part of a mold used to thermoform foam parts. The mold includes two halves, a first half with a forming cavity and a second half with a forming plug. A male form key is reciprocally mounted in the first mold half and reciprocates into and out of the forming cavity in response to engagement with a first drive pin which is reciprocally mounted in the first mold half and moved by engagement with the second mold half. A female form key is rotatably mounted in the second mold half and is rotated between a first and a second position upon engagement with a second drive pin mounted on the first mold half.
The method of thermoforming foam parts includes placing a foam sheet between a forming cavity mold and a forming plug mold each including the male form key or the female form key as described above. Prior to thermoforming, the male form key is biased to a retracted position out of a cavity and the female form key is biased to a non-engaging position. The forming cavity mold and forming plug mold are then brought together. As this occurs, the male form key and female mold key are moved into the foam sheet forming an undercut or reverse draft in a part formed from the foam sheet. The forming cavity mold and forming plug mold are then moved apart. As this occurs, the male form key and female form key are moved away from the formed part and the formed part can be stripped from the forming cavity and plug molds while maintaining the undercut.
In another embodiment, the male and female form keys are attached to pistons that are fluid driven. Fluid flow to and from the pistons is controlled by valves actuated by a signal indicating opening or closing of the molds.
Although the present invention is disclosed for matched metal molds and thermoforming foam parts, the invention is applicable to non-matched metal molds and forming parts from solid sheets.